1. Field of the Invention
This invention relates to a curable silicone composition, and a cured product thereof, for use as a protective material, encapsulating material or the like for electrical and electronic parts, such as ICs.
2. Description of the Prior Art
Curable silicone compositions and cured products thereof are in use as, for example, a protective material or encapsulating material for electrical and electronic parts such as ICs. Silicones, however, generally have a low coefficient of thermal conductivity and, especially when used as an encapsulating material for high-capacity devices, cannot dissipate effectively heat generated from the devices. Therefore, the silicone encapsulating materials cannot prevent the influence of heat on solder or devices, and are unsatisfactory in reliability.
In order to improve the thermal conductivity of silicone encapsulating materials, a filler of good thermal conductivity has been incorporated in the encapsulating materials. The filler of high thermal conductivity for such use include, for example, silica powder, alumina powder, silicon carbide powder, silicon nitride powder, magnesium oxide powder, diamond powder, powders of metal such as iron, stainless steel, copper, etc., carbon powder and so on.
Of these fillers, the metal powders, carbon powder and the like are electrically conductive and, hence, cannot be used for IC-encapsulating silicone compositions or the like intended to provide electrical insulation. On the other hand, crystalline powders such as silica powder, alumina powder, silicon carbide powder, silicon nitride powder, magnesium oxide powder, diamond powder, etc. are electrically insulating and have good thermal conductivity. On that point, the crystalline powders are suitable for use as a filler in the silicone compositions. Where the crystalline powder used as a filler contains ionic impurities, however, the ionic impurities may dissolve in water vapor penetrating through the silicone, and the resulting solution may exude to the surface of the device or the like. Consequently, corrosion of the device or the like is accelerated in the long run. On this account, crystalline powders with ionic impurity contents rendered as low as possible have been compounded as a filler in silicone gels, silicone potting materials, etc. used heretofore as an encapsulating material for electrical and electronic parts.
However, silicones used in the conventional encapsulating materials have a high coefficient of thermal expansion and the above crystalline powders have a high hardness. Where a silicone comprising such a highly hard filler is used as an encapsulating material, therefore, the device or wiring encapsulated in the silicone might be broken by abrasion.
Namely, heat generated from devices or the like in electrical or electronic parts in use causes thermal expansion of the silicone encapsulating material. The thermal expansion, and shrinkage upon cooling, will be accompanied by movement of the crystalline powder contained in the silicone encapsulating material. During the movement, edge portions of the crystalline particles may abrade the device or wiring, possibly resulting in failure of the device or breakage of the wiring.